This invention relates to a pressure contact type semiconductor device, and more particularly to a pressure contact type transistor wherein an electroconductive plate member is urged against an emitter electrode formed in a semiconductor substrate for collecting current.
One example of the prior art semiconductor device of the type referred to above will firstly be described with reference to FIGS. 1 and 2. The semiconductor device shown in FIGS. 1 and 2 comprises a N conductivity type semiconductor substrate 11 having a large diameter and made of silicon wafer, for example, A N.sup.+impurity is diffused into one surface of the substrate by any one of known methods to form a collector region. AP conductivity type impurity is diffused into the entire surface of the opposite side of the semiconductor substrate 11 to form a layer 12 which is used as the base region. A N.sup.+diffusion region is formed in the P conductivity type diffusion layer 12 and the N.sup.+diffusion layer is then selectively etched to form protruding N.sup.+diffusion regions 13 which are used as emitter regions. Base electrodes 15 are provided for the base region and emitter electrodes 16 are provided for the protruding portion of the emitter regions 13 by the method well known in the art. For the purpose of reinforcing the brittle semiconductor substrate 11, a reinforcing plate 18 made of strong material having substantially the same coefficient of thermal expansion as that of the semiconductor substrate 11 is secured to one surface thereof constituting the collector region by a solder layer 19. Although the reinforcing plate 18 may be used as a current path and a cooling member, from the standpoint of economy a copper block 21 is secured to the reinforcing plate 18 through a solder layer 20. A plate member 22 made of tungsten, for example, is urged against the emitter electrodes 16 and a copper block 23 is secured to the plate member 22. The semiconductor device described above is put in a doughnut shaped casing made of ceramic to form a pressure contact type transistor. The base electrodes 15 are lead out of the casing through the wall thereof.
The prior art semiconductor device described above has a number of problems as described in the following. For example, the emitter electrodes 16 and the base electrodes 15 have a plane pattern as shown in FIG. 2. More particularly, the emitter electrodes 16 and the base electrodes 15 are arranged alternately and concentrically. Each emitter electrode 16 is divided into four segments in the circumferential direction and respective base electrodes are interconnected through the gaps in respective emitter electrodes. With this electrode construction, due to heat cycle stresses created in the radial direction from the center of the semiconductor substrate 11 will act between the emitter electrodes 16 and the plate member 22. Moreover the stresses are larger in the peripheral portion of the semiconductor substrate than in the central portion thereof. Such heat cycle is created because the temperature of the semiconductor device is about 150.degree. during the operation thereof but -20.degree. C when the device is inoperative. When silicon is used for the semiconductor substrate 11, and tungsten is used for the plate member 22, since the coefficients of thermal expansion of silicon and tungsten are 3 .times. 10.sup.-.sup.6 / .degree. C respectively, when the heat cycle is repeated a substantial relative displacement will occur between the emitter electrodes 16 and the plate member 22. Especially when the base and emitter electrodes are arranged concentrically the stresses are applied in a direction perpendicular to respective electrodes and since the radial spacing between adjacent electrodes is extremely small, the emitter electrodes will be deformed or crushed frequently. The chance of such damage increases from the center toward the periphery of the substrate.
To solve this problem it has been proposed to arrange the electrodes in the radial direction or the direction of stresses caused by the heat cycle as shown in FIG. 3. In this arrangement, the base electrodes 30 are arranged in the radial direction which the emitter region 32 extends into the spaces between the base electrodes 30 from the peripheral portion of the semiconductor substrate 31.
With this construction, however, the contact area between the emitter and base regions is smaller than that of the construction shown in FIG. 1. Moreover, it is not possible to effectively utilize the emitter region although the area of the emitter region has been increased. The contact area between the emitter and base regions may be increased by increasing the number of the base electrodes extending in the radial direction. With such construction it is impossible to extend the emitter region formed between the base regions to points near the center of the semiconductor substrate 31. Accordingly, it is impossible to use the central portion of the semiconductor substrate as the semiconductor device. Even when the emitter region is formed close to the center, the spacing between the elctrodes becomes extremely small thus causing short circuiting Such difficulty increases as the number of base electrodes extending in the radial direction increases.